Many power inductors, including those used in power converters and EMI filters, and transmitter coils and receiver coils in wireless power transfer (WPT) systems, are required to operate at high frequencies in a range from 10 KHz to few hundreds of MHz. To achieve better efficiency, the windings of such inductors are required to be carefully designed. Since magnetic materials' performance at such a higher frequency is not good or a significant magnetic field is need in a given space such as the space required in a WPT system, air core inductors are commonly employed.
One drawback of an air core inductor is it may cause significant magnetic interference to nearby components. More particularly, by employing the air core inductors, the interference between the air core inductors and the surrounding components can cause significant issues such as disturbing the operation and/or damaging the surrounding components, increasing power losses caused by induced eddy currents in the adjacent metal components, and/or the like.
FIG. 1 illustrates an implementation of a conventional coil structure. FIG. 1 shows a coil structure having two turns. These two turns 102 and 104 can be implemented as either wires or traces on a printed circuit board (PCB). As shown in FIG. 1, the first turn 102 starts from a first terminal of the coil structure 100 and ends at the starting point of the second turn 104. The second turn 104 ends at a second terminal of the coil structure 100. As shown in FIG. 1, the first turn 102 and the second turn 104 of the coil structure 100 are implemented as two concentric circles. The currents of these two concentric circles flow in a same direction as shown in FIG. 1. The coil structure 100 can be of other suitable shapes such as oval, rectangular and the like.
The coil structure 100 shown in FIG. 1 can provide the desired inductance of a wireless power transfer system. However, a significant portion of the magnetic field of the structure may expand out of the coil structure 100.
FIG. 2 illustrates a magnetic flux distribution of the coil structure shown in FIG. 1. The horizontal axis of FIG. 2 represents the distance from the center of the circles shown in FIG. 1. The unit of the horizontal axis is meter. The vertical axis represents the flux density of the magnetic field generated by the coil structure shown in FIG. 1. The unit of the vertical axis is Tesla. The flux density of the magnetic field is measured at a height of about 1 mm above the top surface of the coil structure 100 shown in FIG. 1. The flux density shown in FIG. 2 is taken along line A-A′ shown in FIG. 1.
As shown in FIG. 2, the flux density has two positive peaks 112 and 116, and two negative peaks 114 and 118. Referring back to FIG. 1, the two turns of the coil structure 100 are immediately next to each other. In addition, the currents flowing through the two turns are in the same direction.
As shown in FIG. 2, slightly away from the peaks of the flux density, the magnetic field generated by the currents flowing through the two turns is not canceled out. As a result, the magnetic flux density in FIG. 2 takes a longer distance to decay to a lower value.
FIG. 3 illustrates another magnetic flux distribution of the coil structure shown in FIG. 1. The horizontal axis of FIG. 3 represents the distance from the center of the circles shown in FIG. 1. The unit of the horizontal axis is meter. The vertical axis represents the flux density of the magnetic field generated by the coil structure shown in FIG. 1. The unit of the vertical axis is Tesla. The flux density of the magnetic field is measured at a height of about 10 mm above the top surface of the coil structure 100. The flux density shown in FIG. 3 is taken along line A-A′ shown in FIG. 1.
The magnetic flux distribution shown in FIG. 3 is similar to that shown in FIG. 2 except that the flux density of the magnetic field is measured at a height of about 10 mm rather than 1 mm above the top surface of the coil structure shown in FIG. 1.
As shown in FIGS. 2-3, since the magnetic flux density takes a longer distance to decay to a lower value, a significant amount of the magnetic flux generated from the coil structure 100 shown in FIG. 1 is outside the coil structure 100. This magnetic field may cut into nearby conductive components, thereby generating power losses and causing interference. Especially, if the coil structure 100 is a WPT coil and another component is a Near Field Communication (NFC) tag. When the NFC tag moves adjacent to the coil structure 100, the components in the NFC tag may be disturbed or damaged by the magnetic field generated by the coil structure 100. It is therefore important to have a coil structure with minimized impact on nearby components.